Due to a large amount of background noise, the communication within a car driving at high or even moderate speeds is often difficult. This is especially true if one of the communication partners is the driver and the other is one of the backseat passengers. As a result of the high noise level, the backseat passengers often lean towards the driver or other front passengers. Furthermore, speakers sometime attempt to overcome the noise levels by speaking louder. Even if both of these responses to the noise enhance the quality of the “communication channel” it is exhausting and uncomfortable for the backseat passengers to continue to lean forward and speak louder for extended periods of time. The situation can be improved by using in-car communication (ICC) systems. These systems receive the speech of each passenger by means of a single microphone or with an array of microphones. The received signals of the speaking passenger are processed by the system and played back via loudspeakers which are located close to the non-speaking passengers. Comparable to public address systems, in-car communication systems operate within a closed electro-acoustic loop. Thus, signal processing is used to guarantee stable operation so as to avoid acoustic feedback which results in artifacts such as howling or whistling.
In-Car Communication systems are implemented to support the communication between passengers in adverse automotive environments. The communication between driver and passengers in the rear seats may be strongly disturbed by background noises caused by the tires of the car, the sound of the engine, wind noises or passing cars. Furthermore, the driver has to concentrate on road traffic and therefore cannot turn toward the listener in the rear seat. Since the driver generally speaks towards the windshield, the sound perceived by passengers in the rear of the car is significantly attenuated. Speech intelligibility is additionally degraded by background noise. Therefore, conventional approaches for In-Car Communication systems use microphones to receive the driver's speech signal perform speech enhancement to reduce background noises and amplify the speech signal before the signal is played back in the rear of the car. The reverse communication channel may be supported as well by using microphones close to the passenger in the rear seat and additional loudspeakers in the front of the car. The first case may be viewed as a unidirectional and the second one as a bidirectional ICC system. Bidirectional systems are conventionally implemented using two ICC instances operated in parallel to avoid unstable amplification within the closed loop of both ICC instances. An “instance” generally means a set of executable processor instructions (e.g., signal processing chain instructions), a software thread, firmware or hardware.
FIG. 1 shows a conventional implementation of a bidirectional ICC system using two ICC signal processing chains. Signal processing includes processing tasks such as noise reduction, equalization and notch filtering (for suppressing feedback). Details of these signal processing tasks can be found in “Signal Processing for In-Car Communication Systems” by Gerhard Schmidt and Tim Haulick, Chapter from “Topics in Acoustic Echo and Noise Control, 2006, Springer Berlin Heidelberg pp 547-598, (referred to herein as “Schmidt and Haulick”) and incorporated by reference in its entirety.
Up to now conventional ICC systems have been successfully implemented only for two acoustic zones, (e.g. for supporting the communication between the front passengers (front zone) and the passengers on the backseat (rear zone)). During operation, sound reinforcement in such a system bears the risk of going unstable due to signal echo coupling back into the microphones. Each ICC processing chain contains two mechanisms to avoid howling artifacts resulting from such instability: a notch filter that suppresses howling artifacts in the signal (e.g., feedback suppression in Schmidt and Haulick, pp. 571-573) and a loss control mechanism to guarantee that both directions are not open at the same time to avoid unstable operation in a closed loop (e.g., see Attenuation Control in Schmidt and Haulick, p. 578). However, when there are multiple acoustic zones, in general the signals that come from one acoustic zone are played back into all other acoustic zones (except for the zone with the active speaker). With M input channels and N output channels there are for each of the M input channels N−1 output channels that have to be supported. This results in M×(N−1) paths that have to be considered and processed. Computations are needed for each acoustic path from each microphone to each loudspeaker to provide specific equalizer settings. In conventional systems this results in M×(N−1) equalizers that have to be computed where M is the number of microphones and N is the number of loudspeakers. In conventional systems, the notch filter is calculated for each signal processing chain. These computations are expensive in terms of processor resources. Therefore the cost to support multiple acoustic zones would be prohibitive for most mass market vehicles.